This invention relates to a silicon oxide powder for use as a negative electrode active material in lithium ion secondary cells, and a method for preparing the same.
Silicon oxide represented by SiO is a well-know material. By taking advantage of its chemical activity, it is used, for example, in the synthesis of industrially useful alkylhalosilanes (Gary N. Bokerman et al., U.S. Pat. No. 5,051,247) and the direct synthesis of siloxanes (Peter L. Timms and William N. Rowlands, EPA 0406000A2 xe2x80x9cPolysiloxane oils and process for their preparationxe2x80x9d). It is also reported that magnesium silicide is prepared by reacting silicon oxide with magnesium at low temperatures (E. Fuglein and U. Schubert, Chem. Mater., 1999, 11, 865-866).
Meanwhile, JP-A 9-7638 corresponding to EP 840386A discloses that the use of SiOx as a negative electrode active material in lithium ion secondary cells facilitates occlusion and release of lithium ions. Silicon oxide is thus expected to find an extra application as the negative electrode active material in lithium ion secondary batteries.
The use of silicon oxide (SiOx) powder as the negative electrode material in lithium ion secondary cells leads to a drastically increased initial capacity, but poor cycle performance.
An object of the invention is to provide a silicon oxide powder which is used as the negative electrode material to construct a lithium ion secondary cell capable of maintaining a high capacity without detracting from cycle performance. Another object is to provide a method for preparing the silicon oxide powder.
We have found that when a silicon oxide powder of the formula: SiOx wherein x has a value within a certain range and having a certain specific surface area is used as the negative electrode material in a lithium ion secondary cell, the cell has a high capacity and excellent cycle performance. The conditions under which a silicon oxide powder of a specific composition having physical properties in the desired range can be prepared have been empirically established.
The invention provides a silicon oxide powder represented by the formula: SiOx wherein x is a number from 1.05 to 1.5 and having a BET specific surface area of 5 to 300 m2/g.
According to another aspect of the invention, the silicon oxide powder is prepared by heating a raw material powder mixture containing at least a silicon dioxide powder in an inert gas atmosphere or in vacuum at a temperature of 1,100 to 1,600xc2x0 C. to generate SiO gas, continuously or intermittently feeding oxygen gas to the SiO gas to form a gas mixture, and depositing the gas mixture on a surface of a cooled substrate.